Air dryer reservoir module components

ABSTRACT

An air dryer and reservoir assembly for providing compressed air from an air compressor (20) on a heavy motor vehicle which includes an air dryer (14) connected to receive compressed air from the air compressor (20) and a secondary reservoir (12), including an integral purge volume (34), for storing compressed air which passes through the air dryer (14), with the air dryer (14) and the secondary reservoir (12) being securely attached to a housing (16) to form an air dryer reservoir module (10). A primary reservoir (18) for storing compressed air from the air dryer (14) is located remote from the air dryer reservoir module (10). A pressure equalizing mechanism (57) is disposed between the primary reservoir (18) and the secondary reservoir (12) for keeping pressure in the two reservoirs (12,18) equal. Components located within the housing (16) control air flow between the air dryer (14) and the primary and secondary reservoirs (18,12) and also monitor the pneumatic brake system circuits fed by the primary and secondary reservoirs (18,12).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Patent applicationSer. No. 08/993,931, filed on Dec. 18, 1997 and now U.S. Pat. No.5,917,139.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to air dryers and more particularly to an airdryer and a reservoir, including a separate purge volume, constructedtogether as a module.

2. Description of Prior Art

Air dryers have been used to dry the air in a heavy vehicle air brakesystem for many years. The advantages of clean arid dry air in air brakesystems has long been recognized, as moisture entrained in the air brakesystem may during cold weather operation cause the components of the airbrake system to freeze, thus rendering the system inoperable. Theseprior art air dryers normally include a desiccant material through whichthe compressed air flows. When the vehicle air compressor has chargedthe vehicle air reservoirs to a sufficient pressure level, thecompressor is unloaded so that the compressor no longer compresses air.When the compressor is unloaded, a signal is transmitted to a purgevalve carried within the air dryer which causes stored compressed air tobackflow through the desiccant at a controlled rate to regenerate thedesiccant. The air used to regenerate the desiccant can be either fromcompressed air stored in an integral purge volume within the air dryeror from compressed air stored in a separate reservoir.

Most prior art compressed air systems for heavy vehicle air brakeapplications use three reservoirs which are separate from the air dryer.A compressor feeds compressed air to the air dryer which after flowingthrough the desiccant bed is fed to a remote supply reservoir. Thesupply reservoir is connected to feed compressed air to a primaryreservoir and a secondary reservoir. The primary and secondaryreservoirs feed pneumatic circuits which can control the air brakesystem of the vehicle. For protection from loss of air pressure theprimary and secondary reservoirs each include a check valve in theirpneumatic connections to the supply reservoir. A pneumatic control lineextends from the supply reservoir to an air pressure governor whichcontrols loading and unloading of the air compressor.

SUMMARY OF THE INVENTION

The present invention is for an air dryer and reservoir assembly, forproviding compressed air from an air compressor for operating the brakesof a heavy motor vehicle, which includes an air dryer connected toreceive compressed air from the air compressor, a secondary airreservoir separate from the air dryer, a housing containing pneumaticcircuit components for controlling the flow of compressed air from theair compressor through the air dryer to the secondary reservoir havingthe air dryer securely attached to one section thereof and the secondaryreservoir securely attached to another section thereof for securing theair dryer and the secondary reservoir together as a unitary air dryerreservoir module. The compressed air brake system with the air dryerreservoir module also includes a primary air reservoir having apneumatic connection to the housing disposed remote from the secondaryair reservoir with the housing containing pneumatic circuit componentsfor controlling compressed air flow between the air dryer and theprimary air reservoir. Pneumatic circuit protection valves are disposedwithin the housing for monitoring the pneumatic circuits fed by theprimary and secondary reservoirs. A failure detection means is alsodisposed within the housing for detecting a failure of the pneumaticcircuits which are fed by the primary and secondary reservoirs and inthe event of a failure of these pneumatic circuits the speed of theheavy vehicle is limited. The herein disclosed air brake system's aircharging and treatment sub-system referred to as the air dryer reservoirmodule provides for a simplified and cost effective air chargingsub-system, using integrated components. The air dryer reservoir moduleis configured in a way that allows the elimination of the supplyreservoir, which is used in the standard three reservoir system, whileproviding a means of simultaneously sensing both the primary reservoirand the secondary reservoir for air compressor and air dryer control.

The air dryer reservoir module integrates components such as pressureprotection valves, the air compressor governor, pressure relief valves,pressure sensors and the like in a way that eliminates the need for thesupply reservoir, the supply reservoir safety valve, several of thestandard three reservoir system's external lines and many of itsfittings. The reduction of components, fittings and pneumatic linesreduces potential failure modes. With respect to a standard threereservoir system, the air dryer reservoir module integrates thefollowing components: air dryer, primary and secondary check valves,supply and secondary drain valves, supply reservoir safety valve, thecompressor governor, auxiliary system pressure protection valves, andthe supply and secondary reservoirs.

A purge volume can be built into the end of the secondary reservoir towhich the air dryer is attached. An internal baffle separates thesecondary reservoir volume from the purge volume. Internal communicationpassages connect the secondary reservoir and purge volume to the airdryer. The dryer reservoir module can also be constructed to equalizethe pressure between the primary reservoir and the secondary reservoir,such that if the vehicle is parked for a given period of time thepressure cannot be replenished. Thereby limiting the use of the vehiclewith a severely leaking reservoir.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the invention reference may be had to thepreferred embodiments exemplary of the inventions shown in theaccompanying drawings in which:

FIG. 1 is an illustration of a prior art three reservoir compressed aircharging system;

FIG. 2 is an illustration of a compressed air charging system using anair dryer reservoir module according to the present invention; FIG. 2Ais an illustration of an inboard view of a portion of the compressed aircharging system using an air dryer reservoir module shown in FIG. 2;

FIG. 3 is a perspective view of an air dryer reservoir module accordingto the present invention;

FIG. 4 is an illustration of an air dryer reservoir module according tothe present invention showing some of its internal pneumatic flow pathsand control components;

FIG. 5 is an illustration of an air dryer reservoir module according tothe present invention used in a service brake system for a heavy motorvehicle;

FIG. 6 is a schematic illustration showing the components included inone embodiment of the air dryer reservoir module; and

FIG. 6A is a view of an air dryer reservoir module that integrates thecomponents shown in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and FIGS. 2 and 3 in particular there isshown an air dryer reservoir module and 10 according to the presentinvention. The air dryer module 10 provides compressed air received froman air compressor 20 for operating the brakes of a heavy motor vehicle.The air dryer module 10 includes an air dryer 14 connected to receivecompressed air from the air compressor 20, a secondary air reservoir 12separate from the air dryer 14, and a housing 16 containing pneumaticcircuit components for controlling the flow of compressed air from theair compressor 20 through the air dryer 14 to the secondary reservoir 12and a primary reservoir 18. The housing 16 has the air dryer 14 securelyattached to one section thereof and the secondary reservoir 12 securelyattached to another section thereof for joining the air dryer 14 and thesecondary reservoir 12 together forming the unitary air dryer reservoirmodule 10. The compressed air brake system with the air dryer reservoirmodule 10 also includes the primary air reservoir 18 having a pneumaticconnection 22 to the housing 16 disposed remote from the secondary airreservoir 12. The housing 16 also containing pneumatic circuitcomponents for controlling compressed air flow between the air dryer 14and the primary air reservoir 18. Pneumatic circuit protection valvesare disposed within the housing 16 for monitoring the pneumatic circuitsfed by the primary reservoir 18 and the secondary reservoir 12. Afailure detection means is also disposed within the housing 16 fordetecting a failure of the pneumatic circuits which are fed by theprimary and secondary reservoirs 18,12 and in the event of a failure ofthese pneumatic circuits the speed of the heavy vehicle is limited.

Referring to FIG. 1 there is shown a relatively standard prior art threereservoir pneumatic charging system for the brake system of a heavymotor vehicle. Most prior art compressed air systems for heavy vehicleair brake applications use a primary reservoir 18, a secondary reservoir12 and a supply reservoir 60 which are separate from the air dryer. Acompressor 20 feeds compressed air to an air dryer 62 which afterflowing through the desiccant bed is fed to the remote supply reservoir60. The supply reservoir 60 is connected to feed compressed air to theprimary reservoir 18 and the secondary reservoir 12. The primaryreservoir 18 and the secondary reservoir 12 feed pneumatic circuitswhich can control the air brake system of the vehicle. For protectionfrom loss of air pressure the primary and secondary reservoirs eachinclude a check valve, 66 and 64 respectively, in their pneumaticconnections to the supply reservoir 60. A pneumatic control line 68extends from the supply reservoir 60 to an air pressure governor 70,mounted on the air compressor 20, which controls loading and unloadingof the air compressor 20. The secondary reservoir 12 also has attachedthereto an 85 psi pressure protection valve 72 and a 55 psi pressureprotection valve 74 which feed accessories on the heavy vehicle. Thesupply reservoir 60 includes a safety valve 76 which is set for 150 psi.All three of the reservoirs 12, 18 and 60 include a manual drain valvewhich can be used for draining liquid from their respective reservoir.

Comparing FIGS. 1 and 2 illustrates the difference between the air dryerreservoir module 10 and the standard three reservoir system. The airdryer reservoir module 10 integrates components, eliminates pneumaticlines and connections as compared to a standard three reservoir systemas shown in FIG. 1. The air dryer reservoir module 10 which is one ofthe air brake system's charging sub-systems is intended for use on heavyvehicles, such as tractors and trucks, and is configured as shown in theFIGS. 2 and 3. Manufacturing of the air dryer reservoir module 10 systemis intended to be by conventional methods. The air dryer reservoirmodule is different from prior art in that it integrates charging systemcomponents in a way that eliminates components, pneumatic lines andpneumatic connections thereby reducing failure modes. The air dryerreservoir module 10 is thought to be the first of its kind to utilizethe ABS/EBS brake system ECU to communicate system failures to theengine ECU to instruct the engine to limit vehicle speed. The air dryerreservoir module 10 concept eliminates the supply tank 60 therebyallowing the primary and secondary service tanks 12 and 18 tocommunicate directly with the integrated air dryer 14 through protectionvalves 35 and 36. This allows the protection valves 35, 36 to beintegrated into the air dryer module 10, reducing external componentsand pneumatic lines.

Referring now to FIGS. 4 and 5, the air dryer reservoir module 10operates as follows. Charge air from the air compressor 20 enters theair dryer reservoir module 10 at its supply port 31, flows through apurge valve 32 to the desiccant bed 33, fills the purge volume 34, flowsthrough a purge orifice (not shown) and flows out a single check valve(not shown). This represents the same charge cycle as used in anAlliedSignal AD-9 or AD-IP air dryer. From the delivery of the singlecheck valve the charge air flows to the supply of the primary protectionvalve 35 and secondary protection valve 36. Charge air pressure buildsuntil the protection valves 35, 36 open, allowing air to flow to theprimary reservoir 18 and the secondary reservoir 12. Charge air pressurecontinues to build until the pressure sensors 37 or a mechanicalgovernor (not shown) reach cutout pressure. At cutout pressure the threeway solenoid 39 or the mechanical governor opens, unloading thecompressor 20 and opening the purge valve 32 purging the air dryer 14.The purge cycle is the same as that of the AlliedSignal AD-9 or AD-IPair dryer. The air dryer reservoir module's 10 integrated componentconfiguration makes it compatible with both integral purge and systempurge type air dryer operation and both mechanical and electrical typegovernors. This allows a single dual function electric solenoid tocontrol the air compressor 20 sending a control signal throughcompressor unloader port 40, unloading function and to purge the airdryer 14 using both the primary and secondary service reservoirs 18 and12 respectively. This configuration also allows both reservoirs 12, 18to purge simultaneously thereby reducing the total system pressure droprequired to regenerate the desiccant bed 33 with systems purge type airdryer operation. Use of pressure protection valves 35 and 36 instead ofsingle check valves 66 and 64 reduces compressor 20 cycling. Without theuse of check valves 64 and 66 primary reservoir 18 and secondaryreservoir 12 are common at pressures above the protection valves 35, 36opening pressure. Therefore pressure differentials do not developbetween primary reservoir 18 and secondary reservoir 12 as air isconsumed. Pressure differentials can develop in standard three reservoirsystems due to improperly sized reservoirs and use of accessory systems,causing the compressor to cutin before the air pressures in bothreservoir 12, 18 have dropped to the cutin pressure.

The air dryer reservoir module 10 can be designed to utilize servicereservoir air to purge the desiccant, i.e., supply purge. The operationof the air dryer reservoir module 10 in a supply purge mode will besimilar to that of an AlliedSignal AD-SP air dryer. The integralpressure protection valves 35,36 will eliminate the need for externallyplumbed protection valves. The internal protection valves 35 and 36,when open will allow air to flow back to a special two position threeway system purge solenoid, located where the three way solenoid 39 is inthe integral purge design. The special two position three way systempurge solenoid will be controlled by the EBS ECU with inputs from thepressure sensors 37 through sensors/solenoid I/O 44. The system purgesolenoid will be designed to communicate a control signal to thecompressor 20 unloader (through compressor unloader port 40) and the airdryer purge valve 32 independently. At cutout pressure the system purgesolenoid will unload the compressor and open the purge valve 32. Thesystem purge solenoid will then close only the purge valve 32 after apredetermined amount of system air is used to purge the dryer'sdesiccant bed 33. At cutin pressure the system purge solenoid will thenload the compressor 20 starting the cycle over.

Referring now to FIG. 6, the embodiment of the illustrated air dryerreservoir module 10 operates as follows. Charge air from the aircompressor 20 enters the air dryer reservoir module 10 at its supplyport 31, fills the purge volume 34, and flows out a single check valve13. Check valve isolates the primary reservoir 18 and the secondaryreservoir 12 from the output of air dryer 14. An over pressure safetyvalve 21 which is set for 150 psi is disposed at the outlet from airdryer 14. From the delivery of the single check valve 13 the charge airflows to the supply of the primary protection valve 35 and secondaryprotection valve 36. Charge air pressure builds until the protectionvalves 35, 36 open, allowing air to flow to the primary reservoir 18 andthe secondary reservoir 12. Charge air pressure continues to build atthe outlet of check valve 13 until a mechanical governor 15, which isconnected to the outlet of check valve 13, reach cutout pressure. Atcutout pressure, which is set at 130 ±5 psi, the mechanical governor 15opens, providing a pressure drop signal through line 19, unloading thecompressor 20 and providing a pneumatic signal along line 17 forinitiating purging of the air dryer 14 with the compressed air containedin purge volume 34. Compressor 20 remains unloaded until the pressure togovernor 15 falls below the cutin pressure, which is set at 105 psi. Atthe cutin pressure the governor 15 closes and the compressor is loadedto again supply compressed air to the inlet 31 of air dryer 14.

The primary protection valve 35 is set to open at 103 ±3 psi and toclose at approximately 95 psi. The secondary protection valve 36 is setto open at 109 ±3 psi and to close at approximately 100 psi. When bothprotection valves 35 and 36 are open the primary reservoir 18 and thesecondary reservoir 12 are in free fluid communication. Secondaryreservoir 12 provides compressed air to a vehicle leveling supply portthrough an accessory protection valve 41 which is set to open at 85 ±3psi and to close at approximately 72 to 83 psi. Secondary reservoir 12also provides compressed air to vehicle accessories through an accessoryprotection valve 43 which is set to open at 55 ±3 psi and to close atapproximately 45 to 55 psi.

The air dryer reservoir module 10 is designed to accommodate therequirements of both FMVSS 121 and EEC regulation No. 13. The air dryerreservoir module 10 is also designed to interact with the ABS/EBSsystem's J1939/J1922 engine serial link to allow the limiting of vehiclespeed in the event of a single circuit brake system failure, eitherprimary or secondary. The air dryer reservoir module 10 reduces thenumber of OEM installed charging sub-system components, pneumatic linesand connections as follows: Major charging system components--four forair dryer reservoir module 10 vs. thirteen for a three reservoir system;pneumatic lines--three for air dryer reservoir module 10 vs. six for athree reservoir system; and pneumatic connections--thirteen for airdryer reservoir module 10 vs. thirty two for a three reservoir system.

FIG. 5 shows the air dryer reservoir module, in a complete service brakesystem. The air dryer reservoir module 10 system can be designed tocommunicate pneumatic circuit failures through the ABS/EBS ECU 50 to theengine serial link (J1922/J1939) to instruct the engine to go into a"limp" mode. The intention of this feature is to provide a "nuisance"motivator to the operator to have the vehicle repaired. The systemoperates as follows: In the event of a failure of a pneumatic systemwhich is supplied by either the primary reservoir 18 or the secondaryreservoir 12 the air dryer reservoir module's 10 integral pressuresensors 37 communicates the system pressure status to the ABS/EBS ECU50. The ECU 50 instructs the engine control module (ECM) 52 to limitvehicle speed if the vehicle is in excessive of a set limit.

Referring now to FIG. 4 there is shown the secondary reservoir 12 withan integral purge volume 34. A baffle 53 separates the purge volume 34from the secondary reservoir 12. An internal tube 54 extends through thepurge volume 34 to connect the reservoir 12 through connections withinhousing 16 to the air dryer 14. A connection 55 connects the purgevolume 34 through connections within housing 16 to the air dryer 14.This construction eliminates external lines for connecting the air dryer14 to the purge volume 34 and the secondary reservoir 12. External lineshave the potential for leak points, and create customer handling andmounting concerns. Building the purge volume 34 required for the airdryer into the secondary reservoir 12 allows the use of a compact systempurge air dryer desiccant cartridge and this minimizes the spacerequired. Baffle 53 has tube 54 attached through it and the tube 54extends through the purge volume 34 and terminates at the head of thereservoir. The head of the reservoir has attached to it the housing 16which is integral with air dryer 14. The air dryer communicates withboth volumes 12 and 34 via separate passages 54 and 55.

With a leaking reservoir 12 or 18 the dryer reservoir module 10described above will recharge the surviving reservoir indefinitely,thereby holding off the spring brakes. The driver can choose to operatethe vehicle with limited braking efficiency for an unlimited period oftime even though one of the reservoirs does not hold pressure. Inanother embodiment the dryer reservoir module 10 can be constructed toequalize the pressure between the reservoirs 12 and 18, such that if thevehicle is parked for a given period of time the pressure cannot bereplenished thereby limiting the use of the vehicle with a severelyleaking reservoir.

As shown in FIG. 6 and described above, pressure protection valves 35and 36 are used to supply compressed air to the primary reservoir 18 andthe secondary reservoir 12. The pressure protection valves 35, 36 areset to pressurize the primary reservoir 18 first, provided the samepressure exists within the primary and secondary reservoirs. At fullsystem pressure the pressure protection valves 35, 36 are open, insuringequal pressure in both the primary and secondary reservoirs 18 and 12.However, the pressure protection valve opening pressure is dependentupon the downstream reservoir pressure, therefore, if the reservoirpressures are not equal the pressure protection valve which has thehighest downstream pressure will open first. Parking the vehicleovernight or for extended periods of time may cause the primary andsecondary reservoir pressures to become unequal. During recharging ofthe air system the secondary reservoir 12 may charge before the primaryreservoir 18. It may be desirable to limit the duration the vehicle canbe operated in the condition where one reservoir has significantlyreduced pressure. By using a pressure equalizing mechanism, such as aconnecting line with a suitable orifice 57, the pressures in thereservoirs 12 and 18 slowly become equal so that during recharging ofthe air system the primary reservoir 18 will charge first. Furthermore,if the primary reservoir 18 has become ruptured or has a severe leak thepressure in both reservoirs 12 and 18 will migrate to 0 psi; therebypreventing recharging of the secondary reservoir 12 since the primaryreservoir 18 will not recharge due to the leak. If the secondaryreservoir 12 is ruptured or has a severe leak, again the pressure inboth reservoirs 12 and 18 will migrate to 0 psi; however, the primaryreservoir 18 can be recharged, but the essential accessories and airsuspension will not be pressurized because the secondary reservoir 12cannot be recharged. The pressure equalizing mechanism 57 will cause thepressure in both reservoirs 12 and 18 to be equal, thereby charging theprimary reservoir 18 first and limiting the use of the vehicle after asevere leak in either the primary or secondary reservoirs 18 and 12.

What is claimed is:
 1. An air supply system for a heavy motor vehiclebrake system comprising:a compressor for supplying compressed air; anair dryer connected to receive compressed air from said air compressorincluding a desiccant bed through which the compressed air can flow toprovide a clean and dry compressed air source for operating the brakesystem; a secondary reservoir; a housing connecting said air dryer andsaid secondary reservoir together as a unitary module; a primaryreservoir located away from said secondary reservoir; control componentsdisposed in said housing for controlling air flow from said aircompressor through said air dryer for charging said primary reservoirand said secondary reservoir with compressed air; a purge volume formedintegral with said secondary reservoir; and, said control componentsalso control air flow from said purge volume through said air dryerdesiccant bed to atmosphere to purge said air dryer.
 2. An air supplysystem as claimed in claim 1 comprising:a pressure equalizing mechanismdisposed between said secondary reservoir and said primary reservoir forequalizing the pressure in said secondary reservoir and said primaryreservoir.
 3. An air supply system as claimed in claim 1 comprising:aprimary pneumatic protection control valve disposed within said housingfor controlling pneumatic pressure fed to said primary reservoir; and, asecondary pneumatic protection control valve disposed within saidhousing for controlling the pneumatic pressure fed to said secondaryreservoir.
 4. A compressed air system for a heavy motor vehiclecomprising:an air compressor providing a source of compressed air; ahousing having an inlet connected to receive compressed air from saidair compressor; an air dryer securely fastened to housing; a secondarycompressed air reservoir securely fastened to said housing forming withsaid housing and said air dryer a module; a primary compressed airreservoir having a pneumatic connection to said housing disposed remotefrom said module; said module including control components andconnections for controlling compressed air flow between said air dryerand said secondary compressed air reservoir and said primary compressedair reservoir; and, a pressure equalizing mechanism disposed betweensaid secondary reservoir and said primary reservoir for equalizing thepressure in said secondary reservoir and said primary reservoir.
 5. Acompressed air system as claimed in claim 4 comprising:a purge volumeformed integral with said secondary reservoir; and, said controlcomponents also control air flow from said purge volume through said airdryer desiccant bed to atmosphere to purge said air dryer.
 6. An airdryer assembly for providing compressed air from an air compressor on aheavy motor vehicle comprising:an air dryer connected to receivecompressed air from said air compressor; a first air reservoir separatefrom said air dryer; a housing containing pneumatic circuit componentsfor controlling the flow of compressed air from said air compressorthrough said air dryer to said first reservoir having said air dryer andsaid first reservoir securely attached thereto for securing said airdryer and said first reservoir together as a module; a second airreservoir having a pneumatic connection to said housing disposed remotefrom said first air reservoir; a first pressure protection valvedisposed within said housing for controlling charging of said firstreservoir; a second pressure protection valve disposed within saidhousing for controlling charging of said second reservoir; and, apressure mechanism disposed between said first reservoir and said secondreservoir for equalizing the pressure in said first reservoir and saidsecond reservoir.
 7. A compressed air system for a heavy vehiclepneumatic braking system comprising:an air compressor for supplyingcompressed air when loaded and for not supplying compressed air whenunloaded; a primary reservoir; a secondary reservoir located away fromsaid primary reservoir; an air dryer connected to receive compressed airfrom said air compressor including a dessicant bed through which thecompressed air can flow to provide a clean and dry compressed air sourceto said primary reservoir and said secondary reservoir for operating thebrake system and being connected to said secondary reservoir to form anair dryer and reservoir module; a check valve disposed between said airdryer and said primary and secondary reservoirs and being part of saidair dryer and reservoir module; a purge volume being part of saidsecondary reservoir for purging said desiccant bed when said aircompressor is unloaded; and a pressure equalizing mechanism disposedbetween said secondary reservoir and said primary reservoir forequalizing the pressure in said secondary reservoir and said primaryreservoir.
 8. A compressed air system for a heavy vehicle pneumaticbraking system as claimed in claim 7 system comprising:a primarypressure relief valve being part of said air dryer and reservoir module,being disposed between the outlet of said check valve and said primaryreservoir, and being set to open when the applied pressure exceeds afirst pressure and to close when the applied pressure falls below asecond lower pressure.
 9. A compressed air system for a heavy vehiclepneumatic braking system as claimed in claim 8 system comprising:asecondary pressure relief valve being part of said air dryer andreservoir module, being disposed between the outlet of said check valveand said secondary reservoir, and being set to open when the appliedpressure exceeds a first pressure and to close when the applied pressurefalls below a second lower pressure.